The speed, capacity, and usage of computers have been growing rapidly for several decades. With the development of multimedia and digital communications, the quantity of data being processed has grown faster than the technology that supports it. These factors have given rise to a critical need for new modalities of high density memory. For example, data bases at major international laboratories such as CERN and medical centers are approaching requirements of petabit storage levels per year. Digital media will soon require terabit formats, (see S. Esener and M. Kryder, WTEC Panel Report 7 (1999) 5.2). It may well be that limitations in storing and retrieving data may ultimately limit progress in this key sector to science and technology.
Recently the inventors have investigated the unique properties of a polymer photonic crystal with respect to applications as a medium for high-density 3-D optical data storage for this expressed purpose, see B. Siwick, O. Kalinina, E Kumacheva, R. J. D Miller and J. Noolandi, Appl. Phys. 90 (2001) 5328. This new class of materials has the unique property that information can be confined in spatially modulated domains with well-defined Fourier components. The spatial order imposed on the data storage process is achieved by using nanocomposite polymers in which the optically active domains are localized in the center of self-assemblying latex particles through a two step (or multi-step) growth cycle. Core particles are formed by an optically active dye-labeled monomer. The cores are then subjected to a second stage of polymerization with a lower glass temperature that serves as the optically inactive outer buffer. The resulting particles show very good uniformity and are essentially monodispersed with respect to radii. This property facilitates the formation of hexagonally close packed films up to mm thickness with a very high degree of order. Upon annealing, the close-packed core-shell particles form a nanostructured material with the fluorescent particles periodically embedded into the optically inert matrix in a close-packed hexagonal structure, see E. Kumacheva, O. Kalinina, L. Lilge, Adv. Mater. 11 (1999) 231. The long range periodic order of these materials is demonstrated in FIGS. 1a), 1b) and 1c).
To store information in these materials, a two-photon laser scanning microscope was used to write information by photobleaching the optically active cores and, under much lower fluence, read out the resulting image. The optical properties of these nanocomposite polymers are essentially binary in nature, i.e., the optically active domains are separated by inactive regions with a square modulated cross section determined by the abrupt interface between the high and low temperature polymer blends. This binary modulation of the polymer's optically properties makes these materials ideal for storing binary information, as can be appreciated from FIGS. 1a), 1b) and 1c). The bleaching of the dye in the well defined spatial domains serves as part of the binary code. The sharp boundaries and high local concentration of fluorescent dye serves to enhance the contrast and readily define the bit storage. Equally important, the writing speeds can support GHz address speeds. Overall, a storage density approaching 1 Terabit/cc was successfully demonstrated as disclosed in B. Siwick, O. Kalinina, E Kumacheva, R. J. D Miller and J. Noolandi, Appl. Phys. 90 (2001) 5328.
Relative to conventional homogeneous storage media, the nanostructured periodic material is shown to increase the effective optical storage density by spatially localizing the optically active region and imposing an optically inactive barrier to cross-talk between bits. The basic principle that provides this unique property is shown schematically in FIG. 2. This feature alone leads to a significant increase in the storage density of nanocomposite polymers relative to homogeneous polymers, (B. Siwick, O. Kalinina, E Kumacheva, R. J. D Miller and J. Noolandi, Appl. Phys. 90 (2001) 5328). The aim of the current work is to explore the possibility of further taking advantage of the periodic spatial properties of these materials.
Conventional limits in optical resolution (Rayleigh criterion) assume no a priori knowledge of the location of an object(s). In the case of periodic materials, the objects of interest are confined to lattice points with a well-defined lattice spacing such that there is additional information available for signal processing. The periodic nature of the signal that would result from such structures is well suited to spatial phase sensitive detection methods, i.e. the underlying lattice period acts as a reference for signal processing. Signal-to-noise enhancements of several orders of magnitude are typically realized in related problems using optical heterodyne detection. The question is how far could the spatial resolution be improved by explicitly taking advantage of the underlying Fourier spatial components to nanocomposite materials. To this end, a post processing algorithm was developed that is optimal for extracting binary information from images of bit patterns in nanocomposite materials at densities far beyond the classic Rayleigh resolution limit. Under realistic noise conditions, this signal processing procedure should lead to a density increase of an order of magnitude.
The storage density limit in optical systems is determined by the resolution of the imaging system. When observed through an optical instrument such as a microscope, the image of an individual bit appears as an intensity distribution. The width of this distribution or diffraction pattern is proportional to the wavelength λ of the light in the imaging system. If the bits are separated by a distance λ, then there will be little interference between the diffraction patterns of the bits. We will refer to this spacing as normal density. As the bit separation is reduced, it becomes harder to distinguish individual bits. When the central maximum of one bit's image falls on the first minimum of a second bit, the images are said to be just resolved. This limiting condition of resolution is known as Rayleigh's criterion. In other words, if the distance between bits is smaller than λ/2 the microscope will not be able to distinguish between two adjacent bits. We will refer to this spacing as the classical limit density.
A CD-ROM that holds a maximum of 650 Mbytes over an area of 50 cm2 (radius 4 cm) has a density of 13 Mbytes/cm2. For such systems which use a 780 nm (infrared) laser the normal density is around 20 Mbytes/cm2. Hence, commercial CD-ROMs operate at about 60% of the normal density. The bit density of a DVD that uses a 650 nm (red) laser is 7.8×108 bits/cm2. For this wavelength the normal density is around 1.64×108 bits/cm2, and the classical limit density, is 9.5×108 bits/cm2. The commercial DVD's operate near 80% of the classical limit density. Although the DVD's density is a great improvement (˜7 times more data per disc) over the older CD-ROM technology, it is still limited by diffraction as expressed in the Rayleigh criterion.
When two diffraction patterns are superimposed, in order to satisfy the Rayleigh criterion, they must be separated by more than half the width of the central peak.                                           r            ij                    >                                    1.22              ⁢              π              ⁢                                                           ⁢              z                                      R              ⁢                                                           ⁢                              k                0                                                    =                                                            1.22                ⁢                z                            R                        ⁢                          (                                                λ                  0                                2                            )                        ⁢                                                   ⁢            since            ⁢                                                                       ⁢                                                                     ⁢            λ                    =                                    2              ⁢              π                        k                                              [        1        ]            where z is x, R is y, and λ0 is z. The minimum resolved separation distance (rij) is therefore proportional to half the wavelength. This relation defines the maximum density that can be stored in the image plane and is responsible for the λ−2 scaling for 2-d optical storage. By taking advantage of confocal imaging to provide a 3rd dimension, the maximum density scales as λ−3. The above relation then defines the storage density for one page in the optical readout for 3-d memory and the confocal parameter defines the density of these pages along the optic axis. The density along the optic axis can be increased to a certain extent by using theta scans (see Steffen Lindek, Rainer Pick, Ernst H K Stelzer Rev. Sci. Instrum. 65, 3367 (1994)) however, the storage density scales quadratically for in-plane components and provides far greater gain in storage density.
It would be very advantageous to have a technique that allows data to be read at resolutions beyond the Rayleigh criterion.